Conventional graphics processors are exemplified by systems and methods developed to read and filter texture map samples. To simplify the texture map filtering performed within a graphics processor, a texture is prefiltered and various resolutions of the prefiltered texture are stored as mip mapped texture maps. FIG. 1A is a conceptual diagram of prior art showing a mip mapped texture including a highest resolution texture map, Texture Map 101. A Texture Map 102, a Texture Map 103, and a Texture Map 104 are successively lower resolution texture maps, mip maps, each storing prefiltered texture samples.
Classic mip maps are isotropically filtered, i.e. filtered symmetrically in the horizontal and vertical directions using a square filter pattern. Isotropically filtered mip maps result in high quality images for surfaces with major and minor texture axes that are similar in length. However, when an isotropically filtered texture is applied to a receding surface viewed “on edge”, aliasing artifacts (blurring) become apparent to a viewer as the texture is effectively “stretched” in one dimension, the receding direction, as the texture is applied to the surface. A Footprint 115 is a pixel footprint in texture space, with a Position 135 being the pixel center. FIG. 1B illustrates a prior art application of Texture Map 101 applied to pixels of a Surface 140 that is receding in image space. When viewed in image space, Footprint 115 (an ellipse) appears as Footprint 116 (a circle). While isotropic filtering of texture samples within a pixel footprint that forms a circle in texture space results in a high-quality image, isotropic filtering of texture samples within a pixel footprint that forms an ellipse, such as Footprint 115, results in an image with aliasing artifacts. In contrast to isotropic filtering, anisotropic filtering uses a rectangular shaped filter pattern, resulting in fewer aliasing artifacts for footprints with major and minor axes that are not similar in length in texture space.
FIG. 1C illustrates Footprint 115 including a Minor Axis 125 that is significantly shorter than a Major Axis 130. FIG. 1D illustrates a prior art application of anisotropic filtering of Texture Samples 150 along Major Axis 130. Texture Samples 150 read from one or more mip maps are anisotropically filtered to produce a filtered texture sample. Classic anisotropic filtering filters 16 texture samples in a non-square pattern, compared with 8 texture samples isotropically filtered when trilinear interpolation is used or 4 texture samples isotropically filtered when bilinear interpolation is used to produce the filtered texture sample. Therefore, anisotropic filtering reads and processes twice as many texture samples as trilinear filtering.
In general, producing a higher-quality image, such as an image produced using anisotropic filtering, requires reading and processing more texture samples to produce each filtered texture sample. Therefore, texture sample filtering performance decreases as image quality improves, due to limited bandwidth available for reading texture samples stored in memory and limited computational resources within a graphics processor.
Accordingly, there is a need to balance performance of anisotropic texture sample filtering with image quality to minimize image quality degradation for a desired level of anisotropic texture sample filtering performance.